Error while sending email with attachment s

hi....

I am sending email from R/3 with .xls file attachment. The program is running fine when the size of the doc. is low. When the size increases the mail is not going and gives error message in the outbox. The return code in the program is zero. I am using FM SO_DOCUMENT_SEND_API1.

When I send the mail through SBWP(Workplace) for the same document size it gives no error.

Where do the problem lie? Is it in the setting or i need to modify the code ?

Thanks

Preeti